The present invention relates to a multilayer structure obtained by adhesion or adherence, in particular molecular adhesion, characterized by controlled internal stresses, and to a method for producing such a structure.
By multilayer structure with controlled stresses, it is understood a structure comprising at least two layers, so-called main layers, having between them tensile or compression stresses. These stresses are determined and controlled depending on the purpose of the structure.
The invention finds applications in the fields of microelectronics, as a substrate or as a stiffener, but also in the fields of micromechanics for manufacturing membrane sensors, for example.
Among the multilayer structure assembled by means of molecular adhesion techniques (wafer bonding), let us mention SOI (silicon on an insulator) structures, as an example. Typically, a SOI multilayer structure includes a thick layer of silicon serving as a support, an insulating layer in silicon oxide and a surface layer of thin silicon, the thickness of which is between a few tens of nanometers to a few tens of micrometers.
The manufacturing of SOI structures generally consists of bringing two silicon wafers into contact by molecular adhesion, one of which at least is covered by a silicon oxide surface layer.
After bringing them into contact, the wafers generally undergo a heat treatment under a controlled atmosphere. The purpose of this heat treatment is to enhance the close contact and therefore the adherence of the wafers.
During the heat treatment, the present materials, in particular silicon in contact with silicon oxide, may impose stresses on each other. These stresses are in particular related to the differences in thermal expansion coefficients, xcex94l/1, of the materials in contact. These differences in the expansion coefficients of materials of the surfaces in contact are also the source of stresses when cooling the closely bound structures.
More generally, it is also known that a SiO2 film on a silicon wafer when it is produced at certain temperatures, has the effect of inducing deformation on the wafer upon its cooling. The relative deformation under the effect of heat, noted xcex94l/1, is the order of 2.6.10xe2x88x926/K for silicon, and of the order of 5.10xe2x88x927/K for silicon oxide (SiO2), produced by thermal oxidation of silicon.
When the oxide film is formed on one face of the silicon wafer, the deformation due to the stresses may be quantified by measuring the deflection at the center of the wafer. Because of the difference in thermal expansion coefficients, a decrease of temperature generates a compression of the oxide film on the silicon wafer. This compression is expressed by a convexity of the wafer. The convexity is all the more marked because the oxide film is thick and it may cause a change in the surface morphology.
The appended FIGS. 1-4 are for illustrating the stresses generated in SiO2 structures produced by conventional methods through molecular adhesion.
FIG. 1 shows a first main layer 10a, or support, as a silicon plate having a thin layer of thermal oxide 20a on its surface.
It is seen that the set formed by the first main silicon layer 10a and the oxide surface layer 20a is bent. The surface of the oxide layer 20a is convex.
Reference number 10b refers to a silicon wafer forming a second main layer the parallel faces of which are planar In the illustrated example, the main layers 10a and 10b initially have thicknesses of the same order of magnitude.
FIG. 2 shows the structure obtained by assembling main layers 10a and 10b. These layers are connected through the oxide layer 20a. The assembly, as mentioned earlier, comprises the molecular bonding of the second main layer of silicon 10b onto the oxide surface layer 20a. This bonding is reinforced by a heat treatment.
It is seen that the structure obtained after assembly virtually does not have any deformation. Indeed, from the moment that the thicknesses of the main silicon layers are of the same order of magnitude, stresses generated by the oxide layer on each of the main layers tend to compensate each other.
The silicon surface film with a SOI type structure is generally a thin film, the thickness of which is adapted to the requirements of electrical insulation of the components, for example. The stiffness of the structure is provided by the thick silicon layer.
Thus, in order to obtain a typical SOI structure from the structure of FIG. 2, one of the main silicon layers should be thinned. The thinning may be performed by means of one of the thinning techniques known in different methods, BSOI (Bonded Silicon on Insulator), BESOI (Bonded with Etch stop Layer Silicon on Insulator). On this point, reference may be made to document (7), the reference thereof is specified at the end of the present description.
When one of the main silicon layers is thinned, it appears that the generated stresses at the interfaces with the silicon oxide layer are no longer compensated.
FIGS. 3 and 4 show structures obtained by thinning of the main layers 10b and 10a, respectively. These structures have a deflection and the surface of the thin silicon layer is convex in each of the cases.
It is seen that the thickness of the main layers and also the thickness of the buried silicon oxide layer, i.e., the oxide layer sandwiched between the main layer and the thin surface layer, are part of the parameters which control the deflection of the finally obtained structure.
As an example, for a buried thermal oxide film 20a with a thickness of the order of 1 micrometer, deflection values are obtained which may be larger than 50 xcexcm when the thin surface film of silicon 10a has a thickness of 25 xcexcm and when the main silicon layer has a thickness of the order of 500 xcexcm. When the thickness of the surface film of silicon is increased to more than 50 xcexcm, the deflection decreases by about 25 xcexcm. This shows the importance of the thickness of the silicon film as compared with that of the oxide film.
A conceivable step for reducing the deformations of the structure would consist of producing a second oxide film on the free face, called the rear face, of the thick main layer of the structure. This step would actually enable the deformation of the plates to be reduced before their bringing into contact. However, in a certain number of applications, it is necessary to remove the rear oxide film. Now, after thinning, if the oxide film is removed from the rear face, it is seen that deformation is restored and finally a deformation of the SOI structure mainly related to the thickness of the oxide film, is obtained.
On this point, reference may be made to document (1) the reference of which is specified at the end of the description.
According to another possibility, illustrated by FIG. 5, an attempt may be made to reduce the effect of the stresses by bringing into contact two main silicon layers 10a, 10b each provided with an oxide film 20a, 20b at the surface, the films being of comparable thickness. However, it is seen that a deformation appears for the structure when thinning one of the layers. Further, as shown in FIG. 5, the initial deflection of both main layers increases the difficulty for bringing into contact the surfaces of the oxide surface layers. This may locally generate areas with poor contact and therefore recesses or defects in the final structure.
The deformation phenomenon described above for a structure combining silicon and silicon oxide layers exists for a large number of pairs of materials. However, the generated deformation may vary depending on the materials brought into contact with each other, and notably on the type of stress which occurs, either a tensile or compression stress.
As an example, as shown in FIG. 6, when a silicon nitride film 30 is deposited on a silicon wafer 10, this coating may generate, depending on the conditions of its implementation and after cooling, stresses also leading to a deformation.
The stresses between the silicon nitride and the silicon intrinsically originate from the materials but also have a thermal origin in relationship with the different thermal expansion coefficients. As an example, the thermal expansion coefficient of a silicon nitride film obtained by chemical vapor deposition (CVD) is of the order of 4.2.10xe2x88x926/K whereas this coefficient is 2.6.10xe2x88x926/K for silicon. As deposition of silicon nitride takes place at a high temperature, strong stresses occur during the cooling.
However, it is seen, by comparing for example, FIGS. 6 and 1, that the surface of the silicon nitride layer 30 is concave, unlike the convex surface of the silicon oxide layer 20a. 
This difference in curvature expresses the fact that silicon nitride and silicon oxide have contact stresses which are generally opposite (tension-compression) when they are produced on the main silicon support.
It is also understood that the combination of two main silicon layers, each covered with a film of silicon nitride, according to FIG. 6, may also pose adherence or contact quality problems when the nitride films are facing each other. In particular, bubbles are likely to form at the interface between the silicon nitride layers, locally generating defects in the final structure.
For a better illustration of the aforementioned problems, reference may be made to documents (2), (3), (4), (5) and (6), the references of which are indicated at the end of the description and which relate to contact stresses between different layers.
Document (3) in particular, shows that it is possible to compensate the effects or stresses generated by a silicon oxide surface film formed at the surface of a silicon wafer, by covering this surface film with a second film of silicon nitride.
A substantially plane structure may be obtained.
The thickness of the second (nitride) film should be accurately controlled in order to finally obtain a structure with plane faces.
However, it is found that the generated stresses between the layers are not simply related to the materials brought into contact, as in the case for layers produced by successive depositions for example, but they are also related to the quality of the molecular adhesion between the layers.
Thus, subsequent treatments undergone by a structure according to document (3), or the combination of such a structure with other layers, may cause a change in the balance of stresses so that the final stresses of the structures are difficult to control.
An object of the present invention is to provide a method for producing a multilayer structure including at least one adhesion step, and providing accurate control of the stresses occurring in the structure after combining layers of different materials.
An object is in particular, to provide such a method for changing and adjusting the stresses in order to obtain a planar final structure or having a predetermined deflection.
An object is to be able to transfer, by utilizing the adhesion, at least a crystalline layer in order to obtain a structure with controlled stress.
An object is also to provide such a method for producing a structure free from contact defects at the interfaces between the layers of different materials.
Still another object is to provide a method able to take into account treatments prior or posterior to the production of the structure and which is compatible with the requirements of an industrial implementation such as for example an implantation treatment in order to obtain a separation.
To achieve these objects, the invention more specifically relates to a method for producing a multilayer structure comprising at least a first and second layers called main layers, connected with each other by a stack of at least two stress adaptation layers and having a determined structure stress, wherein:
a) the first main layer is provided with a first stress adaptation layer, and at least a second stress adaptation layer is provided on one of the second main layer and the first stress adaptation layer,
b) an assembly of the first and second main layers is made via stress adaptation layers (said assembly advantageously comprising an adherence bond between layers), the first and second adaptation layers being produced in materials and with thicknesses such that at the end of the method, said determined structure stress is obtained in the structure.
For example, the first adaptation layer and the second adaptation layer are selected (type of realization, nature, thickness) such as, if they are on the first main layer and on the second main layer, respectively, independently (i.e. before assembly), they cause deformations in opposite directions. These deformations are not necessarily of the same amplitude.
In certain embodiments, at least one of the adaptation layers is surmounted with an intermediate layer in order to obtain the desired multilayer structure.
Advantageously, a heat treatment with sufficient temperature and duration in order to adjust said determined structure stress in the structure, may then be carried out, after step b).
According to a preferred embodiment, the adherence bond may be a molecular adherence type bond.
The invention may also use a bond selected from a braze, a weld, a bond by means of an adhesive substance, interdiffusion between layers or a combination of these different techniques. In these techniques, the bond is produced by means of a so-called bonding layer. This bonding layer is either between the adaptation layers or between one of the adaptation layers and the corresponding main layer.
The structure stress is understood as the stress resulting from the stresses of each of the adaptation layers, the stresses of each of the main layers and the stresses related to the bond interface.
The structure stress determines the deflection, either convex or concave, or the planarity of the surfaces of the obtained structure.
The heat treatment optionally carried out after step b) not only provides an enhancement of the quality of the bond, but especially, by adjusting the implemented heat expenditure, enables the contact stresses between the layers to be changed in order to adjust the tensile and compression stress balance.
The implemented heat expenditure may be adjusted by notably taking into account the heat expenditures of treatments prior or posterior to step c). Thus, other heat treatments performed on the structure are not detrimental to obtaining a given stress.
The heat treatment expenditure is also adjusted according to other parameters controlling the stresses in the layers.
Among these parameters, let us mention:
the implemented materials and the treatments undergone by these materials,
the thickness of the layers and their embodiments,
the roughness condition of the surface and the shape of the layers brought into contact,
the quality of the cleaning of the surfaces and their more or less hydrophilicity.
By taking into account these parameters for selecting the heat expenditure, it is possible to adapt the internal stress of the final structure and therefore its deformation. In particular, stresses in the stress adaptation layers may be increased, decreased or even reversed.
According to a first possibility of implementation of the invention, in step a), the first stress adaptation layer may be formed on the first main layer and the second stress adaptation layer formed on the second main layer. In this case, in step b), a bond is produced between the adaptation layers.
As the contact stresses of the stress adaptation layers with the main layers are of opposite sign, one of the stress adaptation layers has a convex surface and the second adaptation layer has a concave surface.
The surfaces to be assembled thus have, to a certain extent, a shape complementarity which provides a quality contact free from bond defects such as recesses or poorly adhered areas.
According to an alternative, both stress adaptation layers may be formed on the first main layer and the bonding may occur between the second main layer and the surface stress adaptation layer, securely fixed to the first main layer.
According to another aspect of the invention, before step b), a preparation of the layers which are to be associated through a molecular bond may be performed in order to adjust a surface condition of these layers and to impart hydrophilicity to them, for example.
The adjustment of the condition of the surface may either consist of a smoothing process (chemical, mechano-chemical process or by a heat treatment) or, on the contrary, of an operation tending to further roughen the surface of at least one of the layers to be assembled.
By changing the amplitude of the roughness of the faces to be assembled, it is possible to control the adhesion energy between the layers and therefore the resulting stresses.
According to an alternative, the method may include a thinning step of one of the main layers after assembly.
It is advantageous to obtain a thin layer, in particular on a thin silicon layer, above the stress adaptation layers one of which is at least an insulator, for example for the subsequent production of integrated electronic circuits (the SOI substrate for example).
The thinning of one of the main layers may be performed by a mechanical or mechano-chemical abrasive treatment.
Thinning may also be performed by fracture. In this case, the method includes at least an ion implantation of gas species in at least one of the main layers or adaptation layers in order to form a fracture area, and the thinning step includes a separation step for said implanted layer according to the fracture area, for example with a thermal and/or mechanical treatment. The stress of the structure will then be changed by the thinning step. In addition, the stress during the course of the method may advantageously be utilized as a determined xe2x80x9cintermediatexe2x80x9d structure stress, in order to participate in this thinning. The final structure obtained after thinning, i.e. after separating one of the layers, has a new determined xe2x80x9cfinalxe2x80x9d structure stress. The structure according to the invention, in certain alternatives, may contain a certain number of layers, certain of which may be thinned, or even suppressed, their role being justified in certain cases, only for adapting the intermediate stress which participates in the thinning. The adaptation of the intermediate stress may be an object per se. By using an intermediate stress which participates in the separation, the dose of the implanted species (hydrogen and/or rare gases), and/or the heat expenditure and/or the work induced by the mechanical force(s) applied for the separation, may be reduced. For example, it is possible to obtain separation with very low heat expenditure on structures where the main layers have different thermal expansion coefficients. By controlling the intermediate stress, the method may be considerably enhanced by changing either the implantation conditions or the separation conditions.
The making of a fracture area in a layer by implantation of a gas species may be performed according to known techniques per se.
For example, one of the techniques uses an implantation of a gas species able to generate an embrittled layer formed of microcavities or gas microbubbles.
A xe2x80x9cmicrocavity or gas microbubblexe2x80x9d means any cavity generated by implantation of hydrogen gas and/or rare gas ions in the material. The cavities may assume a very flat shape, i.e. with a low height, for example of a few interatomic distances as well as a spherical shape or any other shape different from both of these previous shapes. These cavities may contain a free gas phase and/or gas atoms derived from the implanted ions fixed on the atoms of the material forming the walls of the cavities; these cavities may even be empty.
These cavities are generally called xe2x80x9cplateletsxe2x80x9d xe2x80x9cmicroblistersxe2x80x9d or even xe2x80x9cbubblesxe2x80x9d.
Gas species mean elements, for example hydrogen or rare gases in their atomic form (for example H), or in their molecular form (for example H2) or in their ion form (for example H+, H2+) or in their isotope form (for example deuterium) or isotope and ion form.
Moreover, ion implantation is understood as any kind of means for introducing the previously defined species, either alone or combined, such as ion bombardment, diffusion, etc.
The fracture heat treatment is performed with a thermal expenditure which depends on the thermal expenditure supplied to the main layer during the implantation, and during the steps which took place before the fracture. According to the case, this thermal treatment may be zero in time and/or in temperature. Further, this heat treatment may be adjusted according to other exerted stresses, such as for example, mechanical, tensile, shear, bending forces, etc., either exerted alone or combined.
The heat treatment, regardless of the type of solid material, leads to the coalescence of the microcavities which brings about an embrittlement of the structure at the microcavity layer. This embrittlement enables the material to be separated under the effect of internal and/or pressure stresses in the microcavities, this separation may be natural or assisted by applying external stresses.
Mechanical forces may be applied perpendicularly to the planes of the layers and/or parallel to the latter. They may be localized at a point or an area, or be applied at different locations in a symmetrical or dissymmetrical way.
In addition, if the intention is to adapt the final structure stress, the heat expenditure for the fracture is taken into account for establishing the heat expenditure of the adaptation step. The adaptation step for the stresses of the final structure may also include a thinning step, for example by sacrificial oxidation and/or chemical etching and/or plasma etching and/or polishing.
Several possibilities may be contemplated for producing the stress adaptation layers.
According to a first possibility, at least one of the stress adaptation layers may be formed by depositing material according to a deposition method selected for example from spray, epitaxy, chemical deposition such as chemical phase deposition, low pressure vapor deposition, and plasma-aided chemical vapor deposition methods.
According to an alternative, a stress adaptation layer may also be obtained by a surface oxidation of one of the main layers.
In particular, when one of the main layers is a silicon layer, one of the adaptation layers, may be a thermal oxide layer of SiO2.
According to a third possibility, at least one stress adaptation layer may be obtained by implanting species in a main layer. By implanting species in one of the main layers, an area with changed properties may be formed at the surface of this layer.
In particular, by implanting species, stresses may be generated and the density of the material may be changed locally. The depth at which is located the majority of the implanted species, depends on the implantation conditions, for example, on its energy, if the implantation is of the ion implantation type. The film of implanted species, defined by this depth, and its neighborhood, where the majority of the implanted species is localized, then forms one of the layers of the stress bilayer. The film between this film of implanted species and the surface of the second main layer may form one of the two films of the stress bilayer.
The stress intensity may be adapted depending on the nature of the species, on the dose or on various implantation parameters (temperature, implantation current, energy, . . . ). In particular, the implantation may be performed with gas species, for example hydrogen and/or rare gases.
The presence of a stress in the structure participates in the separation and enables the dose of implanted species (hydrogen and/or rare gases), and the heat expenditure and/or the work induced by the mechanical force(s) applied for separating it, to be reduced. Thus, the presence of this stress enables either the implantation conditions or the separation conditions to be changed. The method may be considerably enhanced by controlling the stress. For example, it provides fracture with very low heat expenditure on structures where the main layers have different thermal expansion coefficients. The implantation may also be performed before or after assembling the structure.
In certain cases where at least one of the adaptation layers is sufficiently thick and/or stiff, the normally adjacent main layer may be omitted or may coincide with this adaptation layer. After separation, a changed main layer and a multilayer stack are obtained, wherein the latter may be reused as a main layer comprising a stress adaptation layer.
The embodiment of the method described above, may be applied to main layers in miscellaneous materials. The main layers in identical or different materials may for example be in monocrystalline, polycrystalline or amorph materials and for example in silicon, germanium, silicon carbide, in type III-V or II-VI semiconductors such as GaAs, GaN, InP, . . . , in glass or quartz, in superconducting materials, in diamond, or in ceramic materials (such as LiTaO3, LiNbO3, . . . ).
Thus, the main layer may be formed by one or several layers for example adhered, coated or epitaxied layers.
The stress adaptation layers for example may be in a material selected from SiO2, SiN, Si3N4, TiN, diamond and metals (such as Pd, alloys, . . . ) or in one of the materials which may form one of the main layers, or in a combination of such materials.
The invention also relates to a multilayer structure with controlled internal stresses comprising, in this order, a stack of a first main layer, of at least a first stress adaptation layer in contact with the first main layer, of at least a second stress adaptation layer in contact with said stress adaptation layer and a second main layer in contact with the second stress adaptation layer. In this structure, the first and second stress adaptation layers have contact stresses with the first and second main layers which are respectively with the opposite sign.
In a particular application, the structure may have a suspended membrane, the suspended membrane including at least a portion of one of the first and second main layers, released from the second main layer, from the first main layer, respectively.
The suspended membrane may support other functional layers. For example, it may further include at least one layer of supraconducting material covering said portion of one of the first and second main layers.
Other features and advantages of the present invention will become more apparent from the description which follows, with reference to the figures of the appended drawings. This description is given as a purely non-limiting illustration.